YIG-Tuned Integrated Devices

Vol. 4 No. 5 September/October 1977

6 IG-Tuned |

Integrated
Devices

+49: WATKINS-JOHNSON COMPANY.

COPYRIGHT @ 1977

The process of integrating standard
microwave components such as oscilla-
tors, filters, mixers, or amplifiers into
subassemblies enclosed in a compact
package produces what might be called
a “supercomponent” such as a fre-
quency converter or receiver front end.
There are a number of potential advan-
tages to the systems manufacturer in
the use of such supercomponents in
place of the individual devices. First is
the elimination of the common practice
of system designers to overspecify a
component’s performance. This over-
specification occurs because of the
need to insure that the system specifi-
cation will be met even if the worst-
case performance ofall the components
occurs simultaneously. A supercom-
ponent manufacturer can trade off
performance among. the various
devices and still meet the requirements
of the assembly. Second, overall per-
formance may improve due to the
common environment of the devices.
The third advantage, of course, is
smaller size and weight. A large frac-

FREQUENCY fy

ali
FILTER

DRIVER

ANTENNA

TUNING
VOLTAGE
COMMAND

tion of the total size of the RF
assemblies is made up of connectors,
cabling, and housings. A super-
component can eliminate much of this
hardware and increase the density of
electrical functions.

Receiver Front Ends

Superheterodyne front ends with
narrow, instantaneous bandwidths are
employed in many surveillance and
spectrum analysis applications. In the
front end shown in Figure 1, the RF
input signal from the antenna passes
through a YIG-tuned filter to a mixer.
The preselector is typically a four-
stage YIG filter with an instantaneous
bandwidth of between 20 MHz and
70 MHz, depending on the operating
frequency. Occasionally, a dual two-
stage filter is used instead to allow the
insertion of an RF amplifier between
the second and third stages. This results
in lower front end noise figure at some
extra cost. The YIG-tuned LO (local
oscillator) is generally a transistor oscil-

TO
DETECTOR

MIXER

| [F
AMPLIFIER

FREQUENCY
NE

Y1G-TUNED | FREQUENCY
OSCILLATOR | '1 *' IF

DRIVER

Figure 1. Typical RF front end.

lator through 8 GHz and a GaAs Gunn-
effect oscillator in X and Ku-bands.
The IF amplifier usually consists of
two transistor stages totalling 30 dB
gain. This, combined with the 6 dB
mixer conversion loss and 4 dB pre-
selector insertion loss, results in a net
RF-to-IF gain of 20 dB. The mixer is a
single balanced quadrature hybrid
requiring LO drive levels of about
+10 dBm. A double balanced mixer is
not required since many of the dy-
namic range problems associated with
single balanced mixers are eliminated
through the use of the preselector.

The isolator between the oscillator and
mixer is used to decrease the mismatch
into which the oscillator must operate.
A large mismatch can result in fre-
quency pulling of the oscillator, or, in
extreme cases, the oscillation may
cease altogether. The isolator in the
RF chain serves to eliminate gain
ripple due to filter-mixer mismatch at
the RF frequency and may also have
the additional benefit of suppressing

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LO radiation at the RF input. In
general, however, the frequency of the
oscillator is sufficiently offset from
the passband of the filter that LO
radiation is not a major problem.

The YIG oscillators and filters com-
mon to these front ends are extremely
linear, magnetically-tuned devices. The
YIG sphere, which serves as the tuning
element, is a resonator whose resonant
frequency is directly proportional to
the magnetic field which surrounds it.
The magnetic field, in turn, is linearily
related to the current used to generate
the field. A typical magnetic circuit is
shown in Figure 2. A pair of coils is
wound around pole pieces made of
high-permeability iron. The YIG
sphere and RF circuit are located in
the gap between the pole pieces. The
outside walls of the magnetic shell
provide a return path for the magnetic
flux, provide for mechanical support
of the RF circuit, and provide shield-
ing from external magnetic fields.

Y1G RESONATOR

Figure 2. Magnetic structure of a YIG-tuned oscillator.

Tracking

Critical to the performance of the
front end is the frequency tracking of
the oscillator and filter. In most
applications the IF frequency is de-
fined by a narrowband IF filter
located after the IF amplifier chain. A
typical IF frequency might be 160
MHz. If the preselector bandwidth is
40 MHz, then a mistracking of 20 MHz
between LO and filter results in a 3 dB
increase in front end noise figure
because the RF signal will be at the
3 dB point on the preselector’s pass-
band.

This mistracking can be caused by a
number of factors, all of which in-
fluence the absolute frequency
accuracy of the devices. Among them
are the small non-linearity in the RF
frequency compared to the tuning
current (tuning nonlinearity), the dif-
ferent relative temperature drifts of
the two devices and the different tem-
perature behavior of the YIG driver
circuits.

Tuning nonlinearity is the deviation in
MHz from the best-fit straight line
tuning curve. Generally, the YIG
devices are very linear, with non-
linearities less than +0.1%. The linear-
ity of YIG oscillators is generally not
as good as that of YIG filters. For
example, in X-band the frequency
error due to non-linearity might be of
the order of +8 MHz. Thus, if the RF
bandwidth of a preselector is 40 MHz
in X-band and a front end is tuned to
10 GHz, the real effective RF band-
width becomes only 24 MHz if the
oscillator non-linearity is +8 MHz. If
there is, in addition, some non-linearity
in the preselector, the effective band-
width will decrease further.

The most serious cause of tracking
problems is the wide temperature
range over which front ends are meant
to operate (typically -54°C to +71°C).
The frequency of a YIG device is
dependent on temperature in two

4

different ways. First, the resonant
frequency of the sphere is temperature
dependent. The temperature depen-
dence is, in turn, a function of the
orientation of the sphere in the field.
Second, the field generated by the
magnetic circuit is also temperature
sensitive. This occurs because the iron
forming the magnetic shell expands
or contracts as the temperature
changes, thereby changing the size of
the gap in which the RF circuit is
located. Since the field varies inversely
with the gap, so does the frequency.
Clearly, the tracking between an oscil-
lator and a filter will degrade as the
temperature varies due to the different
drift characteristics.

The final consideration is YIG driver
drift. A YIG driver circuit is basically
a high input impedance (typically
10K ohms) voltage-to-current conver-
ter which allows a YIG device to be
tuned with a precision voltage source
rather than a current source. For a
fixed input voltage, the output current
which tunes the YIG will vary with
temperature. This drift is different
from one driver to the next due to the
essentially random variation of the
temperature coefficients of the driver’s
constituent resistors, operational
amplifiers, and other components.
Hence, a pair of YIG devices will
generally suffer a relative drift over
temperature due to variations in their
respective drivers.

In conventional systems, several tech-
niques are used to correct mistracking.
One common technique is to wrap the
devices in heater blankets to decrease
the temperature range to which they
are exposed and, hence, limit the rela-
tive drift. This method requires con-
siderable heater power which may not
always be available and also requires
a rather long warm-up time to over-
come the substantial thermal mass of
the devices.

Another technique is to compensate
the device drift through the use of a

temperature sensitive circuit in the
driver. A thermistor can be attached to
the device to sense the temperature
and adjust the driver current in such
a manner as to compensate for the
device’s drift. This involves a rather
complex alignment procedure, adding
significantly to the cost of the front
end.

One effective solution to tracking pro-
blems is to use an integrated YIG-tuned
filter-oscillator. This device provides a
means of control over the relative fre-
quency of the oscillator and filter
without the need to stringently con-
trol the absolute accuracy. The design
objective is to eliminate relative mis-
tracking to as great an extent as pos-
sible, to reduce the size and weight
of the device, to decrease the tuning
power required, and to maintain the
ability to generate an arbitrary IF
offset between the oscillator and
filter. The last criterion rules out a
single pole piece design with both
devices in the same gap.

OFFSET COIL

The design selected is shown in Figure
3. The magnetic circuit consists of a
magnetic shell with two pole pieces,
but a single main tuning coil. The
preselector is mounted in one mag-
netic gap while the oscillator is located
in the other. The main tuning coil
tunes the two devices together across
the frequency band. A small offset-
coil is wound around the oscillator
pole piece only. Current supplied to
this coil generates an offset between
the two devices which can be of
arbitrary magnitude and sign.

Since the oscillator and filter are
housed in a common shell and can be
aligned to have comparable tem-
perature coefficients, mistracking due
to temperature effects is clearly not
a problem. Furthermore, the depen-
dence on driver accuracy is signifi-
cantly reduced. The drift, or accuracy,
of the main coil driver does not affect
the relative frequeneies of the two
devices. The tracking accuracy is
governed solely by the offset coil

OSC elATOK POLE PIECE

PIETER POLE PIECE

MAIN TUNING COIL

Figure 3. Design layout of a W-J Y1IG-tuned integrated filter-oscillator circuit.

driver. This driver is generating offset
frequencies of the order of 100 MHz
to 200 MHz, whereas a typical driver
is usually required to generate current
corresponding to frequencies of the
order of 1 GHz to 20 GHz. Since in
either case a driver-related frequency
error of about 1 MHz can be tolerated,
a 1% error in the ‘“‘differential”’ driver
is allowed, while a .01% error in the
“absolute” driver is required. The
excellent tracking accuracy of a
Watkins-Johnson 1-2 GHz and 8-12
GHz filter-oscillator is displayed in
Figure 4.

Physically, these devices measure
approximately 2.55 x 14 x 1.57
inches in P, L and S bands and 2.2 x
1.8 x 1.5 inches in C, X and Ku bands.
Tuning sensitivities are nominally
18 MHz/mA in the four lower bands,
and 25 MHz/mA in X and Ku band,
reducing the tuning current required
by about 30% compared to the stan-
dard separate units (see Figure 5). A
typical tuner utilizing an integrated
filter-oscillator is illustrated in Figure 6.

GAIN - dB

2000

is FIGURE

l
3000 4000

An Integrated Front End

The next step in supercomponent inte-
gration is that of integrating the mixer
and IF amplifiers with the filter-
oscillator into a complete super-
heterodyne front end. The goal, of
course, is to eliminate the isolators and
connectors used in a_ conventional
superheterodyne receiver in favor of a
compact, lightweight RF-to-IF con-
verter. Additionally, the integration of
all these devices allows the use of
complementary components rather
than over-specified separate devices.
Of necessity, a system designer must
specify for worst case conditions. If,
however, for example, the worst case
filter insertion loss occurs at a fre-
quency where the mixer conversion
loss is very good, the combination may
be able to meet the overall system
specifications even though the individ-
ual components would not meet their
specifications. This implies less align-
ment time for the individual com-
ponents and potentially lower cost for
the integrated devices.

NOISE FIGURE - dB

FREQUENCY - MHz

Figure 4. Tracking data of W-J Y!G-tuned integrated filter-oscillator devices.

INPUT
OSCILLATOR
FILTERED OUTPUT
A 500
A5 All MICROSTRIP

Pp OUTEUT

MIXER

Figure 5. Physical appearance of the standard components (in separate packages) of a
Y1G-tuned front end.

Figure 6. Typical tuner utilizing an integrated filter-oscillator device.

The most serious technical problem in
the construction of a YIG integrated
front end is the effect of eliminating
the isolators between the mixer and
preselector and between the mixer and
LO. The mixer chosen for design of a
W-J S-band integrated front end is the
WJ-M26 biasable mixer, which provides
a minimum of 15 dB L-to-R isolation.
It also provides low RF input VSWR,
typically less than 1.5:1 from 2.0 to
3.9 GHz and less than 1.7:1 from 3.9
to 4.0 GHz for an IF frequency of less
than 300 MHz. The L-port VSWR was
typically 2:0; 125) To improve on’ this
VSWR, a 3 dB attenuator was included
in thin film on the mixer input port,
reducing the effective VSWR to about
1.4:1. The IF amplifiers selected are a
WJ-A71 first stage and a W4J-A5d
second stage. The A71 is a low noise,
high gain amplifier with a 2 dB noise
figure, 18 dB typical gain, and -3 dBm
output power at 1 dB gain compres-
sion. The Ad second stage has about
14 dB gain and a +7 dBm, 1 dB gain
compression point.

170
165
160

Le 88

164
162
160

158
156

OSCILLATOR FREQUENCY,
FILTER FREQUENCY - MHz

1.0

70 7 10 4
FREQUENCY - Ghz

The first integrated front end that was
built was an S-band unit with an RF
coverage of 2.0 to 4.0 GHz and a
160 MHz IF frequency. The goal was a
17 dB noise figure and 18 dB RF-to-
IF gain.

The preselector and LO outputs attach
to a substrate with two 50 ohm lines.
The mixer can be replaced with a sub-
strate containing a 50 ohm line which
connects, either in the oscillator or the
filter, directly to the IF port. This way
both YIG devices can be pre-aligned
without the mixer. After pre-alignment,
the mixer is inserted and the final align-
ment performed on the mixer RF port
and preselector to eliminate VSWR
induced ripple. Connecting the IF
amplifiers is a_ relatively straight-
forward procedure, since the frequency
is only 160 MHz.

The electrical performance of the first
W-J integrated front end is shown in
Figure 7. Noise figure was approxi-
mately 0.5 dB better than the design

li.2 12.0

lL} 2.0
LINEAR FREQUENCY

Figure 7. Electrical performance of a W-J YIG-tuned integrated front end.

goal, while gain proved to be about
2 dB better. The effective RF band-
width was better than 15 MHz. Fre-
quency accuracy was better than
+5 MHz over a 0° to +60° tempera-
ture range. The unit is housed in a
compact structure approximately 2.6
x 2.6 x 2.0 inches, not including the
drivers. YIG-tuned integrated front
ends and filter-oscillators appear as
shown in Figure 8.

Manufacturing experience has shown

Se integrated BY
Pe EE e250

ted Front

that integration of multiple RF devices
can improve system performance and
reliability, and can be cost effective
when produced in sufficient quantities.
And, they compare favorably with
subassemblies composed of separate
components, with the added benefit
of smaller size and weight. Super-
components are especially beneficial
to the system designer who may have
specific “black box’’ requirements for
a certain subassembly, but hasn’t the
manpower to realize these require-
ments in house.

Figure 8. The integration of YIG-tuned front ends and filter-oscillator circuits diminish

the overall size of subassemblies.

References

1. Crescenzi, E.J., R.W. Oglesbee, and R.A. Chappel. “‘Integrating Components
for New Front-End Design,” Microwaves, Vol. 13, No. 8 (August 1974), 35.
2. Gilbert, K. and N.P. Albrecht. “A YIG Filter/Oscillator Integrated Device for
YIG-Tuned Receiver Applications,” Watkins-Johnson Company Application

Note, (August 1970).

3. “Supercomponents Come of Age,” Microwaves, Vol. 15, No. 12 (December

1976), 36.

4, Tokheim, R.E. and J.C. Hoover. “The Four-Port YIG Filter,’ Microwave
Journal, Vol. 13, No. 9 (September 1970).

G

Authors:

James C. Papp

Dr. Papp is Head of the Watkins-
Johnson Company YIG _ Device
Engineering Section. He is responsible
for all product development and
design of YIG devices, including band-
pass and band-reject filters, harmonic
generators, and transistor and bulk-
effect oscillators. He is also respon-
sible for engineering support for YIG
devices being built by the Solid State
Division’s production department.
Since becoming the Head of YIG
engineering in ~- August, 1976, the
section has developed 2 to 6 GHz, 2 to
8 GHz and high power X-band and 18
to 40 GHz Gunn effect oscillators. In
addition, band-reject filters in P
through Ku bands have been developed
and put into production. YIG driver
electronics have been redesigned into
thick-film hybrid form, thereby signi-
ficantly reducing costs. In addition, a
closed loop system allowing very
accurate tracking of a YIG filter to a
reference signal has been developed.

Prior to his assignment as Head, YIG
Device Section, Dr. Papp was a Mem-
ber of the Technical Staff and project
Engineer on a program to develop the

10

set of thin film YIG tuned transistor
oscillators for the F-15 aircraft. The
outgrowth of this program has been
the development of a line of high per-
formance thin film oscillators covering
2 to 12 GHz. He has also been in
charge of development of integrated
tracking filter/oscillators in all fre-
quency bands from 0.5 to 18 GHz, as
well as the development and production
of a complete integrated front end
incorporating filter, oscillator, mixer,
and IF amplification. In addition,
Dr. Papp has served as assistant pro-
gram manager during the pilot pro-
duction phase of the Wild Weasel pro-
gram, which involved the manufacture
of an integrated assembly of oscillators,
harmonic generators, microwave
amplifiers and mixers used in an air-
craft direction finding system, and was
also involved in the development of
the first 26-40 GHz YIG oscillator.

Dr. Papp holds an A.B. and Ph.D. in
physics from the University of Califor-
nia, Berkeley. He is a member of the
American Physical Society, IEEE, and
Phi Beta Kappa.

Robert “‘Val’’ Jackson

Mr. Jackson is an applications engineer
for the Watkins-Johnson Company
Signal Sources, Solid State Division.
As applications engineer for the YIG
Devices product line, he has sales and
marketing responsibility for the
southern portion of the United States.

Formerly, Mr. Jackson was a project
engineer with the Scotts Valley Solid
State Division, and was engaged in
project engineering of ferrite isolators
for internal use by the Scotts Valley
and Palo Alto Divisions of the Watkins-
Johnson Company. His primary assign-
ment was production supervision and
incorporation of design changes and
manufacturing techniques to meet pro-
duction schedules. In the isolator pro-
grams, Mr. Jackson’s responsibilities
included design changes, environmental
testing, component failure studies, and
supervision of personnel within the
production group.

During a four-year tour of duty with
the U.S. Navy, Mr. Jackson attended
both basic and advanced Airborne
Electronic Warfare schools, with
emphasis in_ electronic counter-
measures. His primary assignment was
the operation, modification and main-
tenance of various airborne electronic
warfare systems used on a variety of
naval aircraft. Mr. Jackson was respon-
sible for the operational performance
and reliability of these systems during
a one year tour of duty in Vietnam
while assigned to the Naval Air Wing.
He received numerous awards and
decorations for his performance and
support of electronic warfare during
his military years.

Mr. Jackson holds a B.S.E.T. from the
Oregon Institute of Technology, and is
a member of the Association of Old

Crows, Electronic Defense Association,
IEEE, and Phi Theta Kappa.

11

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